Coefficient of β-stabilization of titanium alloys

Abstract

The coefficient of[3-stabilization KI3 was introduced in Russia for titanium alloys in the seventies. It makes it possible to determine the class of the alloy from its chemical composition and evaluate approximately the mechanical properties. The present paper concerns the effect of various elements and the cooling rate in quenching on KI3. The limits of application of the coefficient of 13-stabilization are determined.The coefficient of 13-stabilization [ 1 ] is determined by the formulawhere c i is the concentration of the 13-stabilizing element in the alloy and cc'ri is its critical concentration.The coefficient of [3-stabilization is arbitrary and is based on unproved assumptions but characterizes the class of the alloy rather accurately. Proceeding from this coefficient, all titanium alloys can be arranged in a sequence in accordance with the stability of the 13-phase. The coefficient KI3 is useful both for metal-science students studying titanium alloys and fbr specialists in the field. The coefficient is very convenient in working with publications concerning newly created alloys.The coefficient of [3-stabilization does not take into account the aluminum present in almost every titanium alloy, the neutral strengthners such as tin and zirconium often contained in the alloys, and the impurities. We will show how this is justified below. Formula (1) for determining the coefficient of ]3-stabilization involves the critical concentration c'c' r, i.e., the minimum possible concentration of [3-stabilizer at which a binary titanium alloy quenched from the single-phase 13-region in water no longer contains martensite (Fig. 1) [2].We begin our consideration with the cooling rate in quenching. In the majority of works devoted to the critical concentration the alloys are subjected to quenching in water at a high cooling rate. This method is convenient technologiAll-Russia Institute of Aircraft Materials, Moscow, Russia.cally, and the cooling rates are close to those used in industry. However, it should be noted that the actual cooling rates in various works differ due to the differences in the size of the specimens and the test conditions. However, the effect of the quenching conditions on the value of c'c' r begins to manifest itself only at very high cooling rates. In [3] the effect of the cooling rate on C'c' r of Ti -Mo alloys has been studied in different quenching media, namely, room-temperature water, water with ice, and liquid nitrogen. After quenching at different rates co' r did not change. The effect of the cooling rate, varied in a wide range, on decomposition of the [3-phase in Ti -Nb alloys with formation of martensite has been studied in [4]. As the cooling rate ofa Ti-17.5 at.% Nb alloy was increased from 0.1 to 1000 K/sec the temperature of the martensite transformation decreased from 350 to about 280~ At a quenching rate of 3000 K/sec the martensite transformation did not begin and the alloy hardened to "pure" 13-phase. 499

Enthalpies of Solution in Ti–X (X = Mo, Nb, V and W) Alloys from First-Principles Calculations

“…Figure 4 shows a comparison of the theoretical metastable equilibrium concentration at 0 K from the first-principles data, o c 0 , with the experimental critical concentration, c cr . 50,51) We found a good linear correlation between the experimental critical concentration and the theoretical metastable equilibrium concentration. In Fig.…”

“…Figure 5 shows a comparison of the theoretical metastable equilibrium concentration at 298 K from the CALPHAD data, c 0 , with the experimental critical concentration, c cr . 50,51) In the case of Ti-based alloys, the difference between c cr and c 0 is negligible (c cr µ c 0 ) as discussed above. However, we found a poor agreement between the experimental critical concentration and the theoretical metastable equilibrium concentration at 298 K from CALPHAD data.…”

“…However, the enthalpies calculated without the entropy effects at 0 K corresponded well with the experimental critical concentration, as will be shown below. Figure 3 shows the schematics of a binary phase diagram for a TiX alloy, 49,50) and the Gibbs free energy of the ¡-and ¢-phases at room temperature (298 K), as a function of alloy composition. The ¡-transus is the boundary between the single-phase ¡-region and the ¡ + ¢-region, and the ¢-transus is the boundary between the ¢-region and the ¡ + ¢-region, as shown in Fig.…”

“…¢-alloys, including stable and metastable ¢-alloys, are defined as alloys that retain 100% ¢-phase upon quenching from temperature above the ¢-transus. 49,50) These alloys contain enough ¢-stabilizing elements to prevent cooling through the martensite start line (M s ), consequently preventing the formation of martensite, such as hexagonal martensite (¡A), at room temperature. The critical concentration c cr is the minimum possible concentration of ¢-stabilizer at which a binary Ti alloy quenched from the single-phase ¢-region in water no longer forms martensite.…”

Mechanical and electrochemical characterization of Ti–12Mo–5Zr alloy for biomedical application

“…However, Zr addition does retard the martensitic transformation during cooling, thereby contributing to the hardenability [16]. Antipov and Moiseev [17] further commented that the addition of 6%Zr could produce an equivalent effect to that of 1.5%Mo. According to the coefficient of ␤ stabilisation, the stability of ␤ phase is quantified as:…”